In the past, the use of metal has been generally avoided in food packages intended for use in microwave ovens. Many attempts have been made to solve the problem of arcing. Recently, it has been discovered that metal shielding can be advantageously used in a food package intended to be heated in a microwave oven. For example, a shielded cylindrical shaped container may be used to produce desirable differential heating in food. A brownie may be heated in an unshielded portion of the package, while ice cream in a shielded zone of the package remains frozen. Metal foil is used to create a shielded zone in the package by wrapping the foil around the cylindrically shaped container.
So much attention has been directed to the problem of arcing that it has overshadowed other problems which have heretofore gone unrecognized. Even in a food package that does not arc when heated in a microwave oven, the addition of metal shielding to food packages has produced some unexpected packaging failures. Such failures include melting and discoloration of the packaging materials. It has been discovered that such failures tend to occur near the edges of the metal shielding.
In the past, packaging materials have been selected for use in a microwave oven based upon the heat stability of the packaging material. Typically, such packaging materials had a rate of heating in the microwave oven that was low relative to the food being heated in the microwave oven. The length of time for microwave heating, of course, is determined by the particular food substance involved. During this predetermined period of time for microwave heating, depending upon the food material that was being heated, typical packaging materials would have a rate of heating which was insufficient to cause adverse effects upon the packaging material itself. Materials selected for packaging should not melt, soften, deform or decompose due to the temperature of the heated food product. In the past, this was the primary concern for the selection of microwavable packaging materials.
However, the use of metal shielding in a package intended for heating in a microwave oven creates additional unexpected concerns. It has been discovered that the metal foil creates electrical fields, particularly concentrated around the edge of the metal foil, that intensifies the electrical field strength in the microwave oven. This intensification can be as great as 50 times the average field strength. In the intense fields which may occur when metal shielding is employed in a food package during microwave cooking, some packaging materials which normally have insignificant microwave absorption have been observed to melt or decompose in a matter of seconds.
The problem of melting packaging materials arises when foods are heated which present a low load to a microwave oven. When a meat roast is placed in a microwave oven, the oven will be so heavily loaded that the problems discussed above are unlikely to be observed. However, when low load foods such as small amounts of breadstuffs, frozen vegetables, or brownies are placed in a microwave oven, problems with packaging materials in close proximity to metal packaging components or shielding can be acute.
Therefore, the need has existed for the definition of packaging materials which are compatible with metal shielding in a microwave environment. Packaging materials must be selected which can be used in close proximity to metal shielding and which will remain stable during a reasonable heating time in a microwave oven.